Most blood cells derived from the bone-marrow are known to possess only a limited number of heavily sialylated glycoproteins. We have recently isolated a major sialoglycoprotein on leukocytes and found that this glycoprotein, termed leukosialin, is ubiquitously present on various human leukocytes, granulocytes, monocytes/macrophages and T- and B-lymphocytes. Our studies showed that leukosialin is significantly glycosylated by O-linked oligosaccharides (90 chains/molecule). The structures of those O-linked oligosaccharides are characteristic to each cell lineage and maturation stage. The polypeptide portion of these molecules are, however, apparently the same, with a molecular size of 52 KDa. So it will be interesting to explore the possibility that leukosialin expresses different functions by having different O-glycosylation in a variety of hematopoietic cells.